1. Field of the Invention
Hitherto, an essential constituent of commercial detergents for use in machine washing processes, for example in domestic dishwashing machines, has generally been sodium metasilicate in admixture with sodium tripolyphosphate (also known as pentasodium triphosphate and referred to hereinafter as STP). Soda and waterglass in particular are used as further constituents along with other components designed to enhance the granulating and/or detergent effect. The mixtures are made up as free-flowing agglomerates, the properties of the product having to meet a number of requirements.
The detergent mixtures are generally strongly alkaline and hence take the breath away. Accordingly, the presence in the product of dust-like fines, as might be expected where powder-form raw materials are used, has to be strictly avoided. In addition, products of this type with a high proportion of fines tend to cake or clump on contact with water in the dispensing compartment of the dishwashing machine, so that there is no guarantee of a sufficiently short dispensing time.
In addition to dispensability and freedom from dust, other important evaluation criteria are the powder density of the detergents and their stability in storage. The powder density should be above 900 g/l to enable the quantity of product required for the washing process to be readily introduced into the dispensing compartment. Since the free-flowing agglomerates contain water, it is necessary during processing of the formulation to ensure that the water remains bound in substantially crystalline form to prevent the granulates from caking in storage.
Commercial detergents are presently produced by two processes, namely, either by mixing granulation, or by mixing up of granulated dust-free raw materials.
Mixing granulation in the presence of water involves a number of difficulties which necessitate careful control of the process. In water-moist granulation, various components of the mixture (particularly STP, anhydrous metasilicate and soda) compete with one another to bind the free water present. Thermodynamically the most stable composition may possibly be achieved only during storage of the products through variable raw-material properties or process conditions which have not been strictly observed. The migration of the water required to that end is generally accompanied by clumping or caking of the product.
The flexibility of mixing granulation in regard to the formulations used is also relatively poor, particularly in one specific direction, namely: the replacement of relatively large amounts of the STP by the finely divided, crystalline zeolite NaA desirable for environmental involves difficulties. In particular, the products obtained in this case are often too low in density and have unsatisfactory dispensing properties. Where mixing granulation is carried out in corresponding machines with high energy consumption, for example Lodige mixers, the product sticks to the mixer walls so that the mixer has to be cleaned at regular intervals. Soda and waterglass have to be used as granulation aids although they make no significant contribution towards the effectiveness of the detergent formulation.
Although the production of mixed products in accordance with the prior art avoids some of the above-mentioned disadvantages of mixing granulation, it has hitherto still been necessary to use pregranulated and hence very expensive raw materials to produce ultimately dust-free mixed products.
2. Discussion of Related Art
Earlier German patent application No. 36 24 336 relates to a process for the production of granular, free-flowing alkaline detergents based on sodium metasilicate in intimate admixture with pentasodium triphosphate and/or finely divided crystalline zeolite NaA as enhancing builders and, if desired, other auxiliaries for an enhanced granulating and/or detergent effect by means of a mixing process, the process being characterized in that the starting components of the mixture are mixed with one another in powder form, the resulting mixture is compacted under high pressure in a roll gap and the compactate obtained is size-reduced to the desired grain size. The pressure limits beyond which no further compaction is obtained are not significantly exceeded in the roll gap. More particularly, the material to be granulated is converted under pressure in the roll gap of a pair of rolls rotating in opposite directions at substantially the same peripheral speed into a sheet-form compactate which is then size-reduced to the desired grain size. It is possible by this process readily to obtain strongly alkaline detergent mixtures of the type in question here in the form of free-flowing, dust-free products which are distinguished in use by their high specific density and their favorable dispensability in standard dishwashing machines. At the same time, the free-flowing products obtained are stable in storage and, in particular, show no tendency towards caking, even in the event of prolonged storage.
The further development of this teaching within the scope of the present invention is concerned with further simplification of the process in cases where the production of the compactate as a whole involves the use of, in particular, finely powered components of which the uniform incorporation in the mixture and subsequent processing in the roll gap to form the compactate can give rise to difficulties.
It has been found that, for example, the zeolite NaA used as a phosphate substitute can give rise to unwanted process difficulties where it is to be used in the form of a fine, spray-dried powder of the type now commercially available for the production of detergents containing zeolite NaA. The use of a generally spray-dried zeolite material such as this can be problematical, particularly in industrial plants, on account of its poor flow properties and its very fine powder structure, resulting in for example dosing problems, the blockage of powder paths or the extraction of fines via the in-plant dust extraction system. In addition, poorly-flowing powders with a tendency to cake can also cause difficulties. Occasionally they are not fully taken in by the rolls, so that the rolls have to be force-fed by a screw or similar machine. However, force feeding can be undesirable precisely in the case of formulations which contain, for example, hydrated metasilicates or other components fusible or plasticizable at relatively low temperatures, because in that case the material to be compacted is in danger of partial plasticization under the effect of the general increase in temperature in the compression zone of the tamping screw, resulting in caking in the region of the screw and on the walls. This is often accompanied by a further increase in temperature through increased friction which can result, for example, in damage to organic components or in the undesirable release of water of crystallization.
An object of the present invention is to produce a granulated detergent composition having good free-flow and dispensing properities and high stability in storage using inexpensive raw materials while at the same time reducing the disadvantages of water-moist mixing granulation. In addition to the processibility of hitherto standard formulations for detergents of the type in question, the invention also seeks to enable finely divided crystalline zeolite NaA to be used instead of or in addition to STP and other components, for example detergency enhancers, to be co-used.
According to the present invention, the afore-mentioned process-related disadvantages can be considerably alleviated through the inclusion of relatively high proportions of already granulated, free-flowing components in the mixture of raw materials.